Colocar tag de paragrafo no texto usando um botão

<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <title>Adicionar tags de parágrafo</title>
  <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css">
  <style>
    textarea {
      width: 100%;
      height: 500px;
    }
  </style>
</head>
<body>
  <div class="container my-5">
    <form>
      <textarea id="textarea"></textarea><br>

        <button type="button" class="btn btn-primary mr-2" onclick="addParagraphTags()">Adicionar tags de parágrafo</button>
        <button type="button" class="btn btn-secondary mr-2" onclick="boldSelectedText()">Negrito</button>
        <button type="button" class="btn btn-success mr-2" onclick="copyText()">Copiar para área de transferência</button>
        <button type="button" class="btn btn-info" onclick="addHyperlink()">Hyperlink</button>

    </form>
  </div>
  <script>
    function addParagraphTags() {
      var textarea = document.getElementById("textarea");
      var text = textarea.value;
      var paragraphs = text.split("\n");
      for (var i = 0; i < paragraphs.length; i++) {
        paragraphs[i] = "<p>" + paragraphs[i] + "</p>";
      }
      textarea.value = paragraphs.join("\n");
    }

    function boldSelectedText() {
      var textarea = document.getElementById("textarea");
      var start = textarea.selectionStart;
      var end = textarea.selectionEnd;
      var selectedText = textarea.value.substring(start, end);
      var newText = textarea.value.substring(0, start) + "<b>" + selectedText + "</b>" + textarea.value.substring(end);
      textarea.value = newText;
    }

    function copyText() {
      var textarea = document.getElementById("textarea");
      textarea.select();
      document.execCommand("copy");
      alert("Texto copiado para a área de transferência");
    }

    function addHyperlink() {
      var textarea = document.getElementById("textarea");
      var start = textarea.selectionStart;
      var end = textarea.selectionEnd;
      var selectedText = textarea.value.substring(start, end);
      var url = prompt("Informe a URL");
      var textarea = document.getElementById("textarea");
var text = textarea.value;

var start = textarea.selectionStart;
var end = textarea.selectionEnd;
var selectedText = text.substring(start, end);

var newText = text.substring(0, start) + "<a href='" + url + "' target='_blank'>" + selectedText + "</a>" + text.substring(end);
textarea.value = newText;
}
</script>
</body>
</html>